This invention relates to electron devices comprising a thermionic cathode and, more particularly, to devices using broad area thermionic cathodes for producing large area electron beams.
Large area electron beams are used in many applications including, for example, curing of paints and the like, as well as in various types of lasers. Thermionic cathodes have emerged as the leading candidate for the electron emission source of large area electron beams for use especially in high power lasers and the engineering concept, and design of the heater structure for such thermionic cathodes is of prime importance for their successful implementation. Present heater structure designs are expensive and difficult to scale to large area. Thus, for example, high peak temperatures in ceramic insulator components reduce system lifetime and thermal runaway conditions result in low reliability. Such present thermionic cathodes are inefficient, expensive, complex, have low reliability and cannot operate at high temperatures for long periods of time.
Thermionic cathodes in accordance with the present invention will operate reliably at high temperatures for long periods of time and are of simple design and inexpensive to produce. This is accomplished by the provision in an evacuated enclosure of a fully impregnated dispenser cathode member or the like which is initially heated by any suitable means to a temperature sufficient for low level electron emission from its rear surface. A heating body or hot plate member of preferably equal size is disposed behind the cathode and can either be part of or the means for initially heating the cathode member or it can be heated with the cathode member to the aforementioned cathode member's rear surface temperature corresponding to a low emission level.
A voltage is applied between the cathode and hot plate member sufficient to draw a current comprising electron flow from the cathode member to the hot plate member across the space separating them. This current flow or back electron beam results in heating of the hot plate member to a temperature sufficient to raise the closely spaced cathode member to, and then maintain it at, the desired front surface emission temperature and simultaneously allow timely termination of the initial heating process since it is only needed initially.
In its general aspect, the present invention has the objective of overcoming the aforementioned disadvantages of prior art directly and indirectly heated thermionic cathodes.
It is another object to provide large area thermionic cathodes.
It is another object to provide a large area thermionic cathode having a new and novel heating arrangement substantially insensitive to thermal runaway conditions.
A further object is to provide a large area thermionic cathode having uniform electron emission that operates reliably and efficiently at high temperatures for long periods of time.